fix(diagnostic): virtual_lines diagnostic columns (#32703)

When multiple diagnostics appear on a single line, the virtual lines for
all diagnostics except the first were rendered with progressively fewer
columns.

runtime/lua/vim/diagnostic.lua

@@ -1757,8 +1757,7 @@ local function render_virtual_lines(namespace, bufnr, diagnostics)
         string.rep(
           ' ',
           -- +1 because indexing starts at 0 in one API but at 1 in the other.
-          -- -1 for non-first lines, since the previous column was already drawn.
-          distance_between_cols(bufnr, diag.lnum, prev_col + 1, diag.col) - 1
+          distance_between_cols(bufnr, diag.lnum, prev_col + 1, diag.col)
         ),
       })
     else

test/functional/lua/diagnostic_spec.lua

@@ -2231,6 +2231,110 @@ describe('vim.diagnostic', function()
       eq('Another error there!', result[2][4].virt_lines[1][3][1])
     end)
 
+    it('highlights multiple diagnostics in a single line by default', function()
+      local result = exec_lua(function()
+        vim.api.nvim_buf_set_lines(
+          _G.diagnostic_bufnr,
+          0,
+          -1,
+          false,
+          { 'def foo(x: int, /, y: str, *, z: float) -> None: ...' }
+        )
+
+        vim.diagnostic.config({ virtual_lines = true })
+
+        vim.diagnostic.set(_G.diagnostic_ns, _G.diagnostic_bufnr, {
+          _G.make_error('Error here!', 0, 8, 0, 9, 'foo_server'),
+          _G.make_error('Another error there!', 0, 19, 0, 20, 'foo_server'),
+          _G.make_error('And another one!', 0, 30, 0, 31, 'foo_server'),
+        })
+
+        local extmarks = _G.get_virt_lines_extmarks(_G.diagnostic_ns)
+        return extmarks
+      end)
+
+      --[[
+      |def foo(x: int, /, y: str, *, z: float) -> None: ...
+      |        │          │          └──── And another one!
+      |        │          └──── Another error there!
+      |        └──── Error here!
+      |        ^ col=8
+      |                   ^ col=19
+      |                              ^ col=30
+
+      11 cols between each diagnostic after the first one (10 spaces + |)
+      ]]
+
+      eq(1, #result)
+      local virt_lines = result[1][4].virt_lines
+      eq(8, virt_lines[1][1][1]:len()) -- first space
+      eq(10, virt_lines[1][3][1]:len()) -- second space
+      eq(10, virt_lines[1][5][1]:len()) -- third space
+      eq('And another one!', virt_lines[1][7][1])
+      eq(8, virt_lines[2][1][1]:len()) -- first space
+      eq(10, virt_lines[2][3][1]:len()) -- second space
+      eq('Another error there!', virt_lines[2][5][1])
+      eq(8, virt_lines[3][1][1]:len()) -- first space
+      eq('Error here!', virt_lines[3][3][1])
+    end)
+
+    it('highlights overlapping diagnostics on a single line', function()
+      local result = exec_lua(function()
+        vim.diagnostic.config({ virtual_lines = true })
+
+        vim.diagnostic.set(_G.diagnostic_ns, _G.diagnostic_bufnr, {
+          _G.make_error('Error here!', 0, 10, 0, 11, 'foo_server'),
+          _G.make_error('Another error here!', 0, 10, 0, 11, 'foo_server'),
+        })
+
+        local extmarks = _G.get_virt_lines_extmarks(_G.diagnostic_ns)
+        return extmarks
+      end)
+
+      --[[
+      |1234567890x
+      |          ├──── Another error here!
+      |          └──── Error here!
+      ]]
+
+      eq(1, #result)
+      local virt_lines = result[1][4].virt_lines
+      eq(10, virt_lines[1][1][1]:len()) -- first space
+      eq('├──── ', virt_lines[1][2][1])
+      eq('Another error here!', virt_lines[1][3][1])
+      eq(10, virt_lines[2][1][1]:len()) -- second space
+      eq('└──── ', virt_lines[2][2][1])
+      eq('Error here!', virt_lines[2][3][1])
+    end)
+
+    it('handles multi-line diagnostic message', function()
+      local result = exec_lua(function()
+        vim.diagnostic.config({ virtual_lines = true })
+
+        vim.diagnostic.set(_G.diagnostic_ns, _G.diagnostic_bufnr, {
+          _G.make_error('Error here!\ngot another line', 0, 10, 0, 11, 'foo_server'),
+        })
+
+        local extmarks = _G.get_virt_lines_extmarks(_G.diagnostic_ns)
+        return extmarks
+      end)
+
+      --[[
+      |1234567890x
+      |          └──── Error here!
+      |                got another line
+      ]]
+
+      eq(1, #result)
+      local virt_lines = result[1][4].virt_lines
+      eq(10, virt_lines[1][1][1]:len()) -- first space
+      eq('└──── ', virt_lines[1][2][1])
+      eq('Error here!', virt_lines[1][3][1])
+      eq(10, virt_lines[2][1][1]:len()) -- second line space
+      eq(6, virt_lines[2][2][1]:len()) -- extra padding
+      eq('got another line', virt_lines[2][3][1])
+    end)
+
     it('can highlight diagnostics only in the current line', function()
       local result = exec_lua(function()
         vim.api.nvim_win_set_cursor(0, { 1, 0 })